These experiments will explore the role of experiences during the suckling period in the expression of later spatial learning. Infant rats presented with many nutrient sites during suckling rapidly learn a spatial maze task with similar response-reward contingencies at weaning, while rats presented with fewer sites during suckling acquire such a task much more slowly. The proposed work aims to extend this finding by assessing some of the facets of later behavior and neural development that may be influenced by early experiences. The permanence of early experience effects will be determined by measuring maze performance at 30, 45, or 60 days of age, with the period between weaning and testing spent either in standard individual cages or group housed in large complex-environment cages. The assessment of early experience effects will be extended to include females as well as males. Sensitive periods, during which the young are especially vulnerable to the early experience, will be determined by limiting the time spent in experimental conditions to specific ages. To assess the effects of the early suckling conditions on specific memory components, a maze task will be structured such that working and reference memory components are separated. Changes in brain anatomy resulting from early suckling experience will be explored by measuring weight of various brain areas and by histological analyses. The results of these experiments will provide new insights about how the ongoing behavior of the infant can be altered in response to its environment and how those early changes become expressed in the behavior and nervous system of the more mature organism. These are questions of fundamental importance in fields such as ethology, biopsychology, and developmental neurobiology. Answers to these basic questions could also lead to a greater understanding of human development.